If you are concerned someone has had access to your computer simply change the password. A thirteen numeric/alphabetic password will keep them at bay. Go into System Preferences < System > Users & Groups.

And if it still bothers you do a three pass format of the hard drive and a clean install of whatever operating system you are using.

Hang on to those original install discs like grim death! Using OS X.7 or later make a bootable USB thumb drive before running Installer!